The present invention relates to communication systems. More particularly, the present invention relates to a system and method for generating and updating a transmission-rate reference table based on the received signal-strength measurement and for using this table to more accurately adjust the transmission rate of each station in an IEEE 802.11 wireless local area network (WLAN).
The IEEE 802.11 standard specifies the medium access control (MAC) and physical characteristics for a wireless local area network (WLAN) to support physical layer units. The IEEE 802.11 standard is defined in International Standard ISO/IEC 8802-11, “Information Technology—Telecommunications and information exchange area networks,” 1999 Edition, which is hereby incorporated by reference in its entirety.
The IEEE 802.11 Physical Layers (PHYs) define a plurality of transmission rates based on different modulations and channel-coding schemes so that the transmitter of a frame can choose one of the multiple rates based on the wireless channel condition between the receiver and itself at a particular time. In general, the lower the transmission rate, the more reliable the transmission. Many different algorithms have been proposed in selecting transmission rate but they require some kind of communication between the transmitter and the receiver regarding the link condition. For example, the transmitting STA needs to know the Signal-to-Interference Ratio (SIR) and the frame-error rate for different transmission rates at the receiving end in order to determine the best transmission rate at a given time. Moreover, the IEEE 802.11 standard is silent about the inter-operability between devices from different vendors, which may or may not include the means to select the right transmission rate.
Accordingly, the present invention provides a novel algorithm capable of providing a dynamic rate adaptation based on the Received Signal Strength (RSS) measured from the received frames. The inventive algorithm is totally independent from the receiving side and does not require any change in the current MAC operation of the STA.
The present invention is directed to a system and method of adjusting the transmission rate in a wireless local area network (WLAN) based on the Received Signal Strength (RSS) measured from frames received therein.
One aspect of the invention relates to a method for determining the transmission rate of a mobile station among a plurality of transmission rates and includes the following steps: measuring an average Received Signal Strength (RSS) value from a plurality of incoming frames received by the mobile station; comparing the averaged RSS value to a predetermined reference table having a minimum RSS required for a particular frame length to transmit at one of the plurality of the transmission rates; and, selecting a new transmission rate of the mobile station for a subsequent transmission of a new frame based on the comparison outcome. The method further includes the steps of: determining whether the new frame is a retransmission of an earlier frame; if so, determining whether the new frame has been transmitted more than a predetermined number of times before; and, setting the new transmission rate to a next lower rate irrespective of the comparison outcome if retransmitted more than the predetermined number of times. If the new frame is not the retransmission of an earlier frame, the length of the new frame is determined, then the averaged RSS value is compared to the minimum RSS required for transmitting the new frame at one of the plurality of the transmission rates in the predetermined reference table. Thereafter, the transmission rate of the new frame is set to the rate that corresponds to a largest minimum RSS but smaller than the averaged RSS from the predetermined table.
According to another aspect of the invention, the minimum RSS values of the predetermined reference table are updated depending on whether the transmission of the new frame is successful based on a response frame received thereon. If unsuccessful, the predetermined reference table using the averaged RSS value is updated. If successful, the received signal strength (RSS) from the response frame is measured, and the predetermined reference table is updated if the measured RSS from the response frame is lower than a current minimum RSS value of the predetermined reference table. Further, the predetermined reference table is updated when the measured RSS value is higher than the minimum RSS value for a next higher rate in the predetermined reference table if the new frame is a retransmitted frame.
Another aspect of the invention relates to a system for determining the transmission rate among a plurality of transmission rates and includes a receiver circuit for demodulating an incoming frame; a power-measurement circuit for measuring a Received Signal Strength (RSS) of the incoming frame received therein; a processor, coupled to the power-measurement circuit, for computing an averaged RSS and selecting a new transmission rate for the transmission of a new frame based on a comparison outcome to a predetermined reference table, the predetermined reference table including a minimum RSS required for a particular frame length to transmit at one of the plurality of the transmission rates; and, a memory, coupled to the processor, for storing the predetermined reference table and the averaged RSS for a subsequent retrieval. The system further includes a transmitter circuit for modulating the new frame based on the new transmission rate.
The foregoing and other features and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views.